This invention relates to a method of fabricating spiral wrapped cartridge cases, and more particularly to such a method for fabricating necked-down spiral wrapped cartridge cases.
As is conventional, cartridge cases for certain types of artillary rounds are often made of spiral wrapped construction. More specifically, a spiral wrapped cartridge case typically comprises a solid metal base and a generally cylindric or tapered body rolled from a single sheet of flat sheet metal. The body may be secured to the base in any number of ways. For example, the base may have a circular groove in its inner face and one end of the body may have an inwardly projecting lip formed thereon. A circular collar is fitted within the groove in the base so that a portion of the collar overlies the inwardly extending lip of the body. Upon forming the case within the die, a special tool exerts an axial force on the collar which deforms the collar and locks it in position on the base and deforms it so it positively grips and secures the body lip. In other spiral wrapped cartridge cases, the body may be secured to the base before the body is inserted into the die.
Regardless of the manner in which the body is secured to the base, present day spiral wrapped cartridge cases are typically formed by placing the base and the body into a die cavity. A forming member of solid elastomeric or plastic material (e.g., urethane plastic or the like) is inserted into the body from the open mouth thereof. Axial compression loading is applied to the forming member by a hydraulic ram or the like thus forcing the forming member into the interior of the body and axially compressing the forming member so as to cause it to expand radially thereby to force the body into conformance with the die. The forming pressure exerted by the forming member on the body is of sufficient magnitude (e.g., about 12,000-17,000 psi) so as to form the body generally to the shape of the die and to cause the overlapping edges of the sheet metal body to interlock thus forming a substantially rigid cartridge case. In the fabrication of some configurations of spiral wrapped cartridge cases, a collar forming tool is carried by the forming member so as to exert axial loading on the locking collar to deform it into the base groove and to secure the body lip to the base cap simultaneously with forming the body.
Spiral wrapped cartridge cases have been proven to be highly successful in field and combat use and are much less expensive to manufacture than, for example, drawn brass or steel cartridge cases. Over the years, millions of these spiral wound cartridge cases have been manufactured and used in combat. However, the use of spiral wrapped cartridge cases has been limited to cartridge cases, such as for 105 mm. howitzer and for five-inch naval guns, which are generally cylindrical (they may have a slight taper along their length) or which have a neck nearly the same diameter as the main body of the case. However, in attempting to produce spiral wrapped cartridge cases having a neck diameter substantially less than the diameter of the main body of the case or in attempting to manufacture cartridge cases having a relatively high degree of taper, the above-described manufacturing process for spiral wrapped cartridge cases utilizing a solid elastomeric forming member is not suitable because it is not possible to insert a sufficient quantity of the forming member into the interior of the body through the relatively narrow neck of the case so as to sufficiently fill the body and to apply the required radially outward forming force to the body.
In the 1950's, a process was developed in France by Mr. Leon Heidmann for forming cartridge cases in which an expandable bladder or sack was inserted into the open mouth of the cartridge case and was then pressurized to high pressure levels by hydraulic fluid. As shown in U.S. Pat. Nos. 2,787,973 and 3,043,254, Mr. Heidmann generated high hydraulic pressure levels by dropping a heavy weight onto a piston movable within the cylinder thereby to instantaneously compress the hydraulic fluid within the cylinder to the desired pressure level.
Reference may also be made to such U.S. Pat. Nos. as 381,242, 3,005,431, 3,290,919, 3,635,061 and 3,910,087 which disclose various metal forming processes in which a metal part is formed by means of hydraulic pressure forcing the part outwardly within a die cavity.